The present invention relates to fuel supply control systems for use in internal combustion engines, and more particularly to a system for properly controlling the air-fuel ratio of a mixture of fuel supplied to an internal combustion engine when the engine has shifted from a high-load operation to a low-load operation.
Conventionally, a fuel supply control method for internal combustion engines is known, e.g., from Japanese Patent Application No. 61-294283, which method is applied to a fuel injection device which has a single fuel injection valve arranged in a united portion of an intake pipe at a location upstream of a throttle valve, for injecting fuel into a plurality of cylinders of the engine to thereby reduce the number of valves and hence lower the manufacturing cost of the fuel injection device.
On the other hand, an air-fuel ratio control method is also known, which controls the air-fuel ratio of a mixture supplied to the engine depending upon operating conditions of the engine so as to improve the fuel consumption and emission characteristics, etc., of the engine. According to this known method, as shown in FIG. 9, as the engine is decelerated to shift from a high-load operating condition to a low-load operating condition, the air-fuel ratio is controlled in such a manner that open-loop control is effected for enriching the mixture in the high-load operating region, i.e., a wide-open-throttle region, feedback control is effected in a medium-load operating region, i.e., a feedback control region, to bring the air-fuel ratio to a stoichiometric value, and open-loop control is effected for leaning the mixture in the low-load operating region, i.e., a mixture leaning region, in the mentioned order. Further, the fuel supply to the engine is interrupted in a predetermined decelerating region, i.e., a fuel cut region, shown by the area lower than the dotted line in FIG. 9, which is included within the mixture leaning region.
However, the method has the disadvantage that if the above sequential control is effected when the engine shifts from the wide-open-throttle region to the mixture leaning region within a brief time after staying in the wide-open-throttle region for a long time, since mixture-enriching control has been effected for a long time in the wide-open-throttle region, a large amount of fuel adheres to the inner wall of the intake pipe, throttle valve, etc. at the departure of the engine from the wide-open-throttle region, but only little of the adhering fuel is drawn into combustion chambers while the engine is passing the feedback control region, because the engine stays in the feedback control region for a short period of time. Thus, most of the fuel remains within the intake pipe even when the engine has then left the feedback control region. After the engine has shifted to the mixture leaning region, the adhering fuel is drawn in large quantities into the combustion chambers due to a decrease in absolute pressure within the intake pipe, whereas, in the mixture leaning region, the throttle valve is fully closed or slightly opened and hence the intake air introduced into the intake pipe is small in amount. Consequently, a very overrich mixture is supplied into the combustion chambers. As a result, part of the mixture will not be burnt within the combustion chambers, and the unburnt fuel is emitted from the combustion chambers into the engine exhaust system, thereby causing so-called after-fire within the exhaust system. Particularly, if the engine is provided with a three-way catalyst as an exhaust-gas purifying device in the exhaust system, such after-fire causes an excessive rise in the temperature of the three-way catalyst, thereby deteriorating the performance of same and hence shortening the service life thereof.
Particularly, in an internal combustion engine of the aforementioned type having a fuel injection valve arranged upstream of the throttle valve, the intake pipe has a long span between the injecting location of the injection valve and the combustion chambers so that the amount of fuel adhering to the intake pipe etc. becomes considerably large, thus making the above problem more serious.